The student has a total of 15 observations, but 2 of them are casual and thus aren’t picked up by the project, which is set to only Research Grade or Needs ID. I’m not sure where you’re seeing 12, I’m seeing either 13 or 15 depending on where I look.
The species count is calculated differently in different places on the website. More info here.
That’s a sort of misleading statement. The Explore page always defaulted to only verifiable observations. The change is that most places that used to go to inaturalist.org/observations/username (which shows all observations, not just verifiable) now instead go to www.inaturalist.org/observations?user_id=username. More info here.